System and m ethod for supplying cryogenic refrigeration

ABSTRACT

Various systems and methods for suppling cryogenic refrigeration to supercomputing applications such as quantum computing operations are provided. The disclosed systems and methods are flexible, efficient and scaleable to meet the cryogenic refrigeration requirements of many supercomputing applications. The disclosed systems and methods include: (i) a liquid nitrogen based integrated refrigeration system that integrates a nitrogen refrigerator with a refrigeration load circuit; (ii) a closed loop liquid nitrogen based refrigerator that provides cooling to the refrigeration load circuit via indirect heat exchange between liquid nitrogen in a nitrogen refrigerator and a separate refrigerant in a closed-loop refrigeration load circuit; and (iii) a liquid air based integrated refrigeration system that integrates an air intake system with a refrigerator and a refrigeration load circuit.

RELATED APPLICATIONS

This application claims the benefit of International Application No.PCT/US2020/062665, filed on Dec. 1, 2020, and U.S. ProvisionalApplication Serial No. 62/950,198, filed on Dec. 19, 2019, which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to liquid nitrogen refrigeration, and moreparticularly, to a liquid nitrogen refrigerator configured to providecryogenic refrigeration to a nitrogen refrigerator, directly orindirectly.

BACKGROUND

There are various industrial gas business opportunities for cryogenicrefrigeration systems tailored for supercomputing applications, such asquantum computing operations performed at large data centers. Quantumcomputer memory and processing requirements must be operated atcryogenic temperatures, which often require the refrigeration to besupplied at or near liquid nitrogen temperatures.

What is needed, therefore is an efficient and flexible refrigerationsystem and method for suppling cryogenic refrigeration to arefrigeration load circuit in supercomputing applications eitherdirectly via an integrated arrangement including with a liquid nitrogenbased refrigerator integrated with the refrigeration load circuit orindirectly via a closed loop liquid nitrogen based refrigerator.

SUMMARY OF THE INVENTION

In one aspect, the present invention may be broadly characterized as aliquid nitrogen based refrigeration system integrated with arefrigeration load circuit and associated methods comprising: (1) anitrogen refrigerator having one or more recycle compressors, a warmbooster compressor, a cold booster compressor, a warm turbine, a coldturbine, and a heat exchanger with at least one cooling passage and atleast one recycle passage; and (2) a refrigeration load circuit havingan expansion valve or a liquid turbine; a separator, a buffer tank, anda refrigeration load. The nitrogen refrigerator is configured to receivea source of nitrogen gas as well as a cold nitrogen gas return streamand produce a liquid nitrogen refrigerant stream. The refrigeration loadcircuit is configured to: (a) receive the nitrogen refrigerant stream;(b) expand the nitrogen refrigerant stream in the expansion valve or theliquid turbine; (c) separate the expanded nitrogen refrigerant stream inthe separator into liquid and vapor portions; (d) cool a refrigerationload with the liquid portion of the expanded nitrogen refrigerant streamwhile vaporizing the liquid portion of the expanded refrigerant stream;and (e) return the vaporized stream and the vapor portion of thenitrogen refrigerant stream as the nitrogen return stream to thenitrogen refrigerator. The present integrated liquid nitrogen basedrefrigeration system and associated methods may include various optionalelements and advantages features as generally shown and described belowwith reference to the embodiments illustrated in FIGS. 1-5 and 8-10 ofthe accompanying drawings. Incorporation of one or more of the preferredoptional elements and advantages features very much depend on thecooling requirements of the refrigeration load including the targetrefrigeration temperature and operating pressures of the refrigerationsystem.

In another aspect, the present invention may also be broadlycharacterized as a closed loop liquid nitrogen based refrigeratorcomprising: (1) a recycle compressor; (2) a cold booster compressor; (3)a cold turbine; (4) a primary heat exchanger with at least one coolingpassage and at least one recycle passage; and (5) an auxiliary heatexchanger to cool a separate refrigerant in a closed-loop refrigerationload circuit in via indirect heat exchange between liquid nitrogen inthe refrigerator and the separate refrigerant in a closed-looprefrigeration load circuit. The closed loop liquid nitrogen basedrefrigerator and associated methods may include elements and features asgenerally shown and described below with reference to the embodimentsillustrated in FIGS. 6-7 of the accompanying drawings.

Finally, the present invention may further be broadly characterized as aliquid air based refrigeration system integrated with a refrigerationload circuit and associated methods comprising: (1) an air intake systemhaving a main air compressor and/or a recycle compressor and apre-purifier; (2) a refrigerator having one or more recycle compressors,a warm booster compressor, a cold booster compressor, a warm turbine, acold turbine, and a heat exchanger with at least one cooling passage andat least one recycle passage; and (3) a refrigeration load circuithaving an expansion valve or a liquid turbine; a separator, a buffertank, and a refrigeration load. The refrigerator is configured toreceive a pre-purified and compressed source of air as well as a coldair return stream and produces a liquid air refrigerant stream. Therefrigeration load circuit is configured to: (a) receive the liquid airrefrigerant stream; (b) expand the liquid air refrigerant stream in anexpansion valve or a liquid turbine; (c) separate the expanded airrefrigerant stream in the separator into a liquid portion and a vaporportion; (d) cool a refrigeration load with the liquid portion of theexpanded air refrigerant stream while vaporizing the liquid portion ofthe expanded air refrigerant stream; and (e) return the vaporized airstream and the vapor portion of the air refrigerant stream as the airreturn stream to the refrigerator. The present integrated liquid airbased refrigeration system and associated methods also may includevarious optional elements and advantages features as generally shown anddescribed below with reference to the embodiments depicted in FIGS.11-13 of the accompanying drawings. As indicated above, use of theoptional elements and advantages features in any given embodiment verymuch depends on the cooling requirements of the refrigeration loadincluding the target refrigeration temperature and operating pressuresof the refrigeration system.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present application concludes with claims distinctly pointingout the subject matter that Applicants regard as the invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of an integrated nitrogen refrigerationsystem configured to provide cryogenic refrigeration to a refrigerationload in supercomputing applications at or near the minimum achievabletemperature;

FIG. 2 is an illustration of an embodiment of the integrated nitrogenrefrigeration system of FIG. 1 depicting a medium pressure nitrogenliquefier arrangement;

FIG. 3 is an illustration of another embodiment of the integratednitrogen refrigeration system of FIG. 1 depicting another mediumpressure nitrogen liquefier arrangement suitable for supplying nitrogenrefrigeration at temperatures warmer than the minimum achievabletemperature;

FIG. 4 is an illustration of yet another embodiment of the integratednitrogen refrigeration system of FIG. 1 depicting a low pressurenitrogen liquefier arrangement;

FIG. 5 is an illustration of yet another embodiment of the integratednitrogen refrigeration system of FIG. 1 depicting a high pressurenitrogen liquefier arrangement;

FIG. 6 is a schematic diagram of a closed loop nitrogen refrigerationsystem configured to provide cryogenic refrigeration to a refrigerationload in supercomputing applications at temperatures warmer than theminimum achievable temperature;

FIG. 7 is an illustration of an embodiment of the closed loop nitrogenrefrigeration system of FIG. 6 ;

FIG. 8 is a simplified schematic diagram of an integrated nitrogenrefrigeration system with a trim heater for stabilization of the returngas flow to the nitrogen refrigerator;

FIG. 9 is an illustration of an embodiment of the integrated nitrogenrefrigeration system of FIG. 8 ;

FIG. 10 is an illustration of another embodiment of the integratednitrogen refrigeration system of FIG. 8 and shown coupled to an airseparation unit as the source of gaseous nitrogen;

FIG. 11 is a schematic diagram of a medium pressure integrated liquidair based refrigeration system configured to provide cryogenicrefrigeration to a refrigeration load in supercomputing applications ator near the minimum achievable temperature;

FIG. 12 is an illustration of another embodiment of a medium pressureintegrated liquid air based refrigeration system configured to supplycryogenic refrigeration at temperatures warmer than the minimumachievable temperature; and

FIG. 13 is an illustration of yet another embodiment of a mediumpressure integrated liquid air based refrigeration system configured tosupply cryogenic refrigeration at temperatures warmer than the minimumachievable temperature.

DETAILED DESCRIPTION

Turning now to the drawings, multiple embodiments of the present systemand method for supplying cryogenic refrigeration to a refrigeration loadcircuit in supercomputing applications, such as quantum computingoperations are shown. Most of the embodiments may be characterized as anintegrated arrangement with a liquid nitrogen based refrigerator (SeeFIGS. 1-5 and 8-10 ) or with a liquid air based refrigerator (See FIGS.11-13 ). Alternatively, cryogenic refrigeration system may be configuredas a closed loop refrigeration arrangement (See FIGS. 6-7 ). In each ofthe illustrated embodiments, a common and key feature is the scalabilityof the depicted system wherein the systems can be sized to provide fromas low as about 20 kW of refrigeration to 2000 kW of refrigeration ormore. The specific configuration or arrangement of the nitrogenrefrigerator or liquid air refrigerator can be optimized depending onthe temperature requirements needed to cool the refrigeration load inthe supercomputing application or other cryogenic refrigerationapplications.

Turning now to FIG. 1 , there is shown a simplified schematic of thepresent system and method for supplying cryogenic refrigeration to arefrigeration load in supercomputing applications. The illustratedsystem is a liquid nitrogen based refrigeration system 10 integratedwith the end-use application (i.e. refrigeration load) 20 and includes aheat exchanger 12, recycle compressor(s) 14, turboexpander(s) 16, and areturn gas circuit 18. The specific arrangement of the nitrogenrefrigerator (i.e. heat exchanger 12, recycle compressor(s) 14, andturboexpander(s) 16) as well as the return gas circuit 18 is highlydependent on the temperature requirements needed to cool therefrigeration load 20, which in turn dictates the operating pressures ofthe nitrogen refrigerator 10.

The most efficient and most cost effective manner of providing thecryogenic refrigeration would be to configure the integrated liquidnitrogen based refrigerator to supply nitrogen refrigerant at minimumachievable temperature. The minimum achievable temperature is typicallytied to the pressure of the liquid nitrogen in the refrigeration loop,and which is preferably attained by reducing the pressure of the in thecryogenic refrigeration loop to at or near ambient pressure. At ambientpressure (i.e. about 14.7 psia) the liquid nitrogen is at a temperatureof about 77.3 K, which is generally the minimum available temperaturefor a nitrogen refrigerator that is configured to operate at ambient orhigher pressures. Operation of a nitrogen refrigerator at sub-ambientpressures is not practical nor desired as the potential for airin-leakage could lead to freezing of any moisture, carbon dioxide, andother air contaminants which could lead to failure or under-performanceof the nitrogen refrigerator.

If minimum temperature operation of the nitrogen refrigerator isdesired, the nitrogen refrigeration system must be controlled so thatthe temperature of the vaporized nitrogen exiting the refrigerator isthe minimum available temperature which, as indicated above, occurs whenthe liquid nitrogen is at or near ambient pressure. To motivate thevaporized nitrogen from the cryogenic refrigerator to and through thelow pressure gas return circuit 18 downstream of the refrigeration load,a cold compressor 19 is optionally used. To perform efficiently, thenitrogen refrigerator produces cold liquid nitrogen at high pressure.The most efficient nitrogen refrigerator design would provide the coldnitrogen at high pressures, and in some applications the nitrogen issupplied at or above the critical pressure. For refrigeration supplytemperatures of between about 77.5 K to 79.1 K the nitrogen pressureexiting the refrigeration load is between about 15 psia to about 18psia. At these low pressure levels, a cold blower (not shown) willlikely be necessary Alternatively, if cryogenic refrigeration attemperatures of about 80.1 K is acceptable for the intended application,the pressure can be about 20 psia. In this case, or in similarapplications where the temperature nitrogen refrigeration supply can beeven warmer, the vaporized nitrogen can be returned to the nitrogenrefrigerator without a cold compressor.

The nitrogen stream returning or recycling back to the nitrogenrefrigerator is at its lowest pressure at the warm end of the heatexchanger just before it enters the recycle compressor(s). As indicatedabove, this recycled nitrogen stream should be at or more preferablyabove atmospheric pressure in order to avoid the possibility of airin-leakage, as this would lead to freezing in the nitrogen refrigerator,and possibly create operational problems in the refrigeration loadsystem. The optional cold blowers raise the pressure of the vaporizednitrogen as it exits the refrigeration load system such that the returncircuit pressure to ensure maintained above atmospheric pressure.Depending on the intended application, multiple, redundant cold blowersmay be required to achieve a high reliability often required ofcryogenic refrigeration systems.

Table 1 shows the approximate power consumption to provide refrigerationwith cryogenic liquid at varying design temperatures based on computerbased simulations and models. The relationship between temperature andpressure exiting the refrigeration load is shown in Table 1. As thetarget refrigeration temperature rises, the relative refrigerator powerdemand (i.e. relative power consumption) decreases as is expected fromthe Second Law of Thermodynamics. However, as the nitrogen refrigerantapproaches its critical point, its latent heat begins to decreaserapidly. Note that the critical point temperature of nitrogen is 126.2 Kand its corresponding critical point pressure is 493 psia. This decreasein latent heat in the nitrogen refrigerant means that the refrigerantflow needed to balance the refrigeration load increases commensurately,which explains why the power demand of the refrigeration system ishigher for a target refrigeration temperature of 118 K than it is for atarget refrigeration temperature of 111 K. Hence, the integrated liquidnitrogen refrigeration system of FIGS. 1-5 is preferred where the targetrefrigeration temperature is below about 115 K.

In addition to the operational cost savings realized from the reducedpower requirement, there may also be modest capital cost savings in theintegrated refrigeration system of FIGS. 1-5 as the target refrigerationtemperature is raised above the lowest design values. The lower powerrequirement of the refrigeration system generally means the recyclecompressor(s), the motor(s), the turbine(s), and the heat exchanger willdecrease in load and/or size, which translates to possible capital costsavings. However, where the higher pressure nitrogen stream directlyprovides cooling to the refrigeration load require that the maximumallowable working pressure rating of the equipment in the refrigerationload circuit must be higher, possibly resulting in increased capitalcosts.

Above a target refrigeration temperature of 115 K, a differentrefrigeration concept such as that shown in FIGS. 6-7 should be used.Sensible heat could be provided in addition to latent heat so that theliquid nitrogen flow is decreased in an attempt to reduce the powerconsumption at target refrigeration temperatures above 111 K, butintroduction of sensible heat will introduce other efficiency penaltiesand is generally not beneficial. The concept for providing highertemperature refrigeration, preferably above about 115 K, is shown in theembodiments shown in FIG. 6 and FIG. 7 and described in more detailbelow.

TABLE 1 Total Refrigerator Power Demand for Warmer TemperatureRefrigeration Refrigerant Refrigerant Temp Refrigerant RelativeRefrigerator Fluid (K) Pressure (psia) Power Demand (%) Nitrogen 77.715.3 100 (baseline) 81.8 1.655 94 89.5 50 81 96.8 90 74 110.9 225 60117.7 325 75 Alternate Fluid 150 — 51 Alternate Fluid 200 — 33

As seen in Table 1, the relative refrigerator power demand associatedwith providing refrigeration at 150 K using a refrigerant other thannitrogen is 51% which is only modestly lower in power consumption thanthe relative refrigerator power demand of 60% associated with providingnitrogen refrigeration at 110.9 K. The primary reason for this narrowdifference is the thermodynamic penalty for cooling the alternaterefrigerant using the sensible heat of gas nitrogen in the auxiliaryheat exchanger. In other words, the heat transfer in the auxiliary heatexchanger is thermodynamically very irreversible due to the largetemperature difference at the cold end and small temperature differenceat the warm end. Other less important efficiency penalties also resultfrom the need for a discrete or separate refrigeration circuit for thesewarmer refrigeration temperatures. Conversely, the relative refrigeratorpower demand associated with providing refrigeration at 200 K using arefrigerant other than nitrogen is 33% which represents a somewhat largepower savings compared to the case providing refrigeration at 150 Kusing a refrigerant other than nitrogen (i.e. relative refrigeratorpower demand of 51%). This substantial power savings is indicative ofthe benefit of the associated with warmer target refrigerationtemperatures. Both these cases bear the thermodynamic irreversibilitypenalty resulting from the auxiliary heat exchanger and the indirectheat transfer compared to the direct liquid nitrogen refrigerationsystems shown in FIGS. 1-5 that are designed or configured to be used atcolder target refrigeration temperatures, specifically colder than about115 K.

A key feature of the embodiments shown in FIGS. 1-5 is the nitrogenrefrigeration circuit is integrated with the refrigeration productionsystem. The refrigeration production system in the illustratedembodiments produces liquid nitrogen at the cold end of the heatexchanger. The liquid nitrogen is then supplied directly to therefrigeration load, where it is vaporized and returned to the cold endof the nitrogen refrigerator. By integrating the liquid nitrogendirectly from the refrigerator with the refrigeration load the need foran auxiliary heat exchanger is avoided that would otherwise be requiredto cool a refrigerant in the separate refrigeration circuit. The finitetemperature difference realized in the auxiliary heat exchanger wouldresult in the actual temperature of the separate refrigerant beingsupplied at the refrigeration load being higher than the temperature ofthe liquid nitrogen being supplied directly to the refrigeration load inthe integrated arrangement. Avoiding this penalty is especiallyimportant in cryogenic refrigeration applications having very lowtemperature requirements.

Turning now to FIGS. 2-5 , the embodiments shown in FIGS. 2 and 3 depictcryogenic refrigeration systems 100 employing medium pressure nitrogenliquefier arrangements, whereas FIG. 4 shows an embodiment of thecryogenic refrigeration system 200 employing a low pressure nitrogenliquefier arrangement and FIG. 5 depicts another embodiment having ahigh pressure nitrogen liquefier arrangement 300. The liquefactioncycles shown in FIGS. 2-5 appear very similar to conventional nitrogenliquefiers. For example, the liquefaction cycle depicted in FIG. 2 ismuch like the medium pressure liquefier disclosed in U.S. Pat. No.4,778,497 while the liquefaction cycle depicted in FIG. 4 is similar tothe low pressure liquefier described in U.S. Pat. No. 6,220,053 and theliquefaction cycle depicted in FIG. 5 is similar to high pressureliquefier described in U.S. Pat. No. 5,231,835.

The disclosed embodiments of the nitrogen refrigerator differ from theconventional nitrogen liquefiers in that they lack a warm nitrogen feedgas supply and the nitrogen refrigerators also have no liquid nitrogensubcooler. Unlike a conventional nitrogen liquefier, the nitrogenrefrigerators shown in FIGS. 2-5 are configured for a full recycle ofthe liquid nitrogen refrigerant, albeit in the vapor phase, such thatthe liquid nitrogen subcooler would provide little or no benefit.

Common features of the nitrogen refrigerators illustrated in FIGS. 2-5include a configuration wherein the nitrogen refrigerators produce coldnitrogen at high pressure at the cold end of the heat exchanger 112. Ineach of the illustrated embodiments, the nitrogen refrigerant exitingthe heat exchanger 112 is let down in pressure through a throttle valve125 or optional liquid turbine 135. The nitrogen refrigerant is thendirected to a separator 140 where the minor flashed gas flow 144 isreturned at low or medium pressure to the heat exchanger 112. Thenitrogen vapor from the separator is let down in pressure through avalve 142 only slightly so the separator 140 operates at a slightlyhigher pressure than the buffer tank 150 and refrigeration load circuit.The liquid nitrogen is passed from the separator 140 to a buffer tank150, which directly supplies the liquid nitrogen refrigerant to therefrigeration load 120. The liquid nitrogen level in the buffer tank iscontrolled to balance the refrigeration load 120. This control ispreferably adjusted by adjusting the nitrogen refrigerant production ofthe refrigerator system 100. The return stream 122 from therefrigeration load 120 is saturated vapor or slightly superheatednitrogen vapor. As previously described, the optional cold compressor119 is needed only when the refrigeration supply temperature is requiredto be near its minimum available temperature, between about 77.5 K toabout 79.1 K. The return stream 122 from the refrigeration load 120 iscombined with the nitrogen vapor 152 from the separator 150 andintroduced into a low pressure return circuit 123 (FIGS. 2 and 4 ) or amedium pressure return circuit 127 (FIG. 3 ) of the heat exchanger 112.Note the heat exchanger include a high pressure nitrogen circuit 129 orpassage for the high pressure feed stream 178. Also, a plurality ofaftercoolers 179 may be employed downstream of compressors, as required.

An optional liquid storage tank 160 is also shown in the variousembodiments including those embodiments shown in FIGS. 2-5 . The purposeof the optional liquid storage tank 160 is to hold externally suppliedliquid nitrogen or to hold liquid nitrogen produced from the liquefierof the nitrogen refrigerator. Depending on the cooling requirements ofthe intended application, liquid nitrogen could be produced by thenitrogen refrigerator in a modal operating method that produces excessliquid nitrogen part of the time and/or consumes the stored liquidnitrogen part of the time. This modal operating method may beadvantageous in situations where power costs vary as a function of time.Alternatively, excess liquid nitrogen could be produced by the nitrogenrefrigerator for export as a merchant liquid or for other uses at thecustomer site in addition to meeting the cooling requirements of theintended application.

In some cryogenic refrigeration applications, the use of a modaloperating method for excess liquid nitrogen production and/or liquidnitrogen consumption may require substantial gas storage at the warm endof the cryogenic refrigeration system. Preferably, a plurality of gasreceivers (See e.g. FIG. 9 ) would be configured to store nitrogenmolecules when the liquid nitrogen tank is emptying; and the gasreceivers would supply the nitrogen molecules when the liquid nitrogentank is filling. In some operating scenarios, including operatingscenarios where excess liquid nitrogen is exported a nitrogen producingair separation unit (See e.g. FIG. 10 ) may be required, particularlyduring modal operation when the gas receiver volume or capacity couldbecome impractically or uneconomically large.

Another beneficial feature of the embodiments shown in FIGS. 2-5 is theuse of radial inflow turbines 170, 175. The radial inflow turbines 170,175 used in the illustrated embodiments are capable of high efficiencywithout compromising operating rangeability and are configured tooperate at a pressure ratio of between about 8.5 to 10.0. In order tocool the high pressure nitrogen stream exiting the cold end of the heatexchanger 112 sufficiently for supply of refrigeration at a targettemperature of 95 K to 100 K or below, the nitrogen stream exiting thecold turbine 175 must be lower than about 90 psia and more preferablybetween 80 psia and 90 psia. Preferably, the nitrogen stream exiting theturbine is nearly a saturated vapor or it can be up to 10% liquid oreven several degrees superheated.

With the desired pressure ratio of the cold turbine 175 between about8.5 to 10.0, the high pressure feed stream 178 is approximately 800psia. This high pressure feed stream 178 enhances the efficiency of therefrigeration system 100 for two thermodynamically based reasons. First,the higher pressure stream results in a straighter cooling curve. As itspressure gets higher above the nitrogen critical pressure of 493 psia,the change in heat capacity as it cools is reduced, resulting in lesssevere “kinks” in the cooling curve. In the lower direction, as itspressure becomes subcritical, the cooling curve then has a constanttemperature latent heat zone which creates a very uneven cooling curveand is very thermodynamically irreversible. Second, higher pressurestreams to the turbine are beneficial thermodynamically simply becausethey have higher heat capacities. This simply means they are better ableto recover refrigeration with lower flows, which results in lower flowand power consumption in the recycle compressors.

In the refrigeration cycle depicted in FIG. 2 , both the warm turbine170 and the cold turbine 175 operate at similar pressures. In aconventional nitrogen liquefier, the warm turbine flow is typicallyabout one-half of the cold turbine flow for this cycle. In this case,though, with essentially all the liquid nitrogen produced at the coldend is returned to the nitrogen refrigerator as a cold gas, therefrigeration demand for the warm turbine is comparatively lower. So,the warm turbine flow in the embodiment of FIG. 2 is preferably onlybetween about 10% to about 20% of the cold turbine flow. Because of thisreduced warm turbine flow, elimination of the warm turbine and boostermay even be considered or contemplated for this embodiment. Thedischarge stream 115 from the medium pressure recycle compressor 114 isfed to the warm booster 172 and cold booster 176 in parallel, and theirrespective discharge streams 173 and 177 are recombined to form combinedstream 178 at the highest pressure in the cycle before they enter theheat exchanger 112, which is preferably a brazed aluminum heatexchanger. The low pressure recycle compressor 113 raises the pressureof the combined feed stream made up of the warmed low pressure flash gasstream 144 and return stream 145 from the refrigeration load 120. Asmall make-up flow 111 may be required to compensate for leakage lossesin the turbomachinery. Very low leakage seals, such as dry gas seals,can be employed if desired to minimize this flow. The low pressurerecycle compressor 113 and the medium pressure recycle compressor 114can be combined into a single machine with one motor and one bull-gear,if desired. As the requirements for the target refrigeration temperatureare raised, the corresponding pressure of nitrogen for the refrigerationload circuit also becomes higher. For example, as shown above in Table1, if the target refrigeration temperature is 89.5 K, the pressure ofnitrogen exiting the refrigeration load circuit is about 50 psia. Theseparator 150, if it is used, will also be commensurately higher inpressure. This means that the low pressure circuit is higher in pressureand the power needed from the low pressure recycle compressor 113 isreduced. In this situation, the number of compression stages needed inthe low pressure recycle compressor 113 may also be reduced.

In the refrigeration cycle depicted in FIG. 3 , the target refrigerationtemperature is preferably about 95 K to about 97 K. The pressure fromgas return stream 145 exiting the refrigeration load circuit is as highas the pressure in the exhaust stream from the cold turbine 175. Unlikethe embodiment shown in FIG. 2 , the embodiment of FIG. 3 has no lowpressure gas return stream and no low pressure recycle compressor.

Yet another embodiment and refrigeration cycle is shown in FIG. 4 .There are two main differences in this configuration compared to theembodiment depicted in FIG. 3 . First, the feed nitrogen 136 to the warmturbine 170 is piped from the recycle compressor discharge 115 ratherthan the warm and cold booster streams. Second, the warm booster 172 andcold booster 176 operate in series rather than parallel, with a portionof the recycle compressor discharge shown as stream 138 first compressedin the warm booster 172, then in the cold booster 176. The high pressurestream 178 from the cold booster discharge supplies the cold turbine 175and the cold liquid nitrogen product stream exiting passage 129 at thecold end of the heat exchanger 112. The configuration shown in FIG. 4 ,with its lower pressure ratio across the warm turbine 170, and lowerpressure ratio with higher flow across the warm booster 172 will lead toa lower speed, larger size impeller design for the warm turbine 170 andwarm booster 172. This configuration also makes use of an additionalheat exchange zone (shown as X-2) between the cold turbine takeoff andthe warm turbine return that is not needed in the embodiments depictedin FIG. 2 and FIG. 3 . This is due to the lower temperature range of thewarm turbine 170 in FIG. 4 resulting from its lower pressure ratio.

FIG. 5 shows yet another alternative embodiment of the cryogenicrefrigerator having a liquefaction cycle that provides improvedefficiency compared to conventional liquefaction systems. To accomplishthis, the warm turbine inlet and the cooling product are designed tooperate at a very high pressure such as about 1300 psia, although theoptimum operating pressure likely depends on the heat exchanger 112 andturbomachinery design tradeoffs. The higher pressure within theliquefaction cycle improves the thermodynamic efficiency of the nitrogenrefrigerator by improving the reversibility of the heat exchangertemperature profile and because of the higher heat capacity of the feedstreams. In the embodiment disclosed in FIG. 5 , the cold turbine 175must operate with an exhaust pressure approximately the same as it isfor other liquefiers, so that it is sufficiently cold to cool the liquidnitrogen product stream exiting passage 129 at the cold end of the heatexchanger 112 to the desired target refrigeration temperature. Thismeans that the cold turbine 175 must have a lower pressure feed andpreferably the lower pressure feed 136 is supplied as a portion from thedischarge stream 115 from high pressure recycle compressor 114.

Because the pressure ratio would be too high if the warm turbineexhausted into the cold turbine exhaust circuit, the warm turbineexhaust in the embodiment of FIG. 5 is supplied to a separate,intermediate pressure circuit 124 or passage within the heat exchanger112. This feature provides some design freedom in selecting the desiredexhaust pressure of the warm turbine 170, and the corresponding returnpressure between the medium pressure recycle compressors 113A, 113B andthe high pressure recycle compressor 114.

Similar to the embodiment shown in FIG. 4 , the warm booster 172 andcold booster 176 are fed in series, albeit in reverse order, with theportion of feed stream 138 first directed to the cold booster 176 andsubsequently directed to the warm booster 172. The additional heatexchange zone, shown as X-2 is preferably disposed between the warmturbine exhaust and cold turbine draw in a manner similar to that of thenitrogen refrigerator of FIG. 4 .

As indicated above, if applications where the design requirementsdictate a target refrigeration temperature above 111K and morepreferably at or above 115 K, a closed loop refrigeration concept suchas that shown in FIGS. 6-7 should be used. As seen in FIG. 6 , aseparate refrigeration circuit 202 containing an alternate refrigerantis used to cool the refrigeration load 220. A closed loop nitrogen basedrefrigerator 205 is used to generate the refrigeration that isindirectly transferred to the separate refrigeration load circuit 202.This is done using vapor nitrogen exiting the turboexpander 216, whichis passed through an auxiliary heat exchanger 215 to cool the alternaterefrigerant in separate refrigerant circuit 202.

The alternate refrigerant fluid is selected such that it providesconstant temperature refrigeration using its latent heat. The preferredalternate refrigerant(s) will have its normal boiling point slightlybelow the target refrigeration temperature so that the separaterefrigeration circuit pressure is modestly above ambient pressure,avoiding concerns for air in-leakage. In addition, the criticaltemperature of the alternate refrigerant must be higher than the targetrefrigeration temperature, preferably by a large margin. Circulating thealternate refrigerant at temperatures well below the criticaltemperature means the refrigeration circuit can be operated at amoderate pressure, and the flow rate within the refrigeration circuitwould be relatively low. The preferred alternate refrigerant isnon-toxic and inflammable. It is also desirable that the alternaterefrigerant has the lowest possible greenhouse warming potential.Potential alternate refrigerants and the normal boiling points include:Krypton (119.9 K); R-14 (145.4 K); nitrous oxide (184.7 K); R-23 (191.1K); R-41 (195.0 K); and R-116 (195.0 K).

FIG. 6 shows a simplified schematic of the closed loop liquid nitrogenbased refrigeration system 200 that includes a main heat exchanger 212,an auxiliary heat exchanger 215, recycle compressor(s) 214, andturbine(s) 216, and as well as the associated gas circuits. The separatealternate refrigerant based system 202 is also a closed looprefrigeration system incorporating the auxiliary heat exchanger 215, oneor more pumps 217, and the refrigeration load 220. The pump 217 is usedraise the pressure of the alternate refrigerant after it is condensed inthe auxiliary heat exchanger 215 so that it can be recirculated. A pumpis preferred instead of a gas phase blower because the pump is generallylower cost, requires less power, and generally causes less of athermodynamic penalty. For cryogenic refrigeration applicationsrequiring a high degree of reliability and/or availability, as well asextended rangeability of the refrigeration supply, multiple pumps may beemployed.

The gas nitrogen exiting the turboexpander (i.e. turbine) 216 is thelowest temperature stream in the refrigerator. It directly provides therefrigeration to balance the refrigerant circuit. The flow of theturbine exhaust stream in the liquid nitrogen based refrigerator must besufficiently high and the temperature must be sufficiently cold toprovide the necessary cooling in the auxiliary heat exchanger 215. Theauxiliary heat exchanger 215 is preferably a counter-current heatexchanger that exhibits a large temperature difference at its cold end,where the turbine exhaust stream enters the auxiliary heat exchanger215. The temperature difference of the counter flowing streams in theauxiliary heat exchanger 215 progressively decreases and is tightest atthe auxiliary heat exchanger warm end, where the turbine exhaust streamexits the auxiliary heat exchanger 215. So, the temperature of theturbine exhaust stream exiting the warm end of the heat exchanger limitsthe operating temperatures of the refrigeration system 200.

A more detailed embodiment of the closed-loop nitrogen refrigerator isshown in FIG. 7 . Due to the higher temperature level of therefrigeration load circuit 202, the cold turbine 216 supplies sufficientrefrigeration so there is generally no need for a warm turbine. Theturbine exhaust temperature and flow are optimized to achieve the lowestpower and capital solution. Reduction in cold turbine flow means thatthe cold turbine exhaust temperature must also be reduced in order toprovide the refrigeration demand. The lowest power solution will have asmall temperature difference at the warm end of the heat exchanger 212,which indicates minimized wasted refrigeration. The selection ofpressure levels is very unconstrained, since there is no liquidgenerated in the refrigerator 205. As for the other systems, higherpressures will tend to improve efficiency. Also, the turbine pressureratio should not exceed 8.5-10.0. For this system 200, a turbinepressure ratio lower than about 10.0 yields a significant power savings.The lower pressure ratio requires increased flow, which gives a moreuniform cooling curve in the heat exchanger 212 and improved efficiency(reduces the temperature difference at the cold end). In turndown ofthis and the other refrigeration systems previously described, thepressure levels are decreased. For the best turndown the pressures ofthe recycle compressor and the turbine are decreased such that thepressure ratio across each are held constant and the volumetric flowsare also constant. In this way the recycle compressor 214A, 214B andturbine 216 each maintain their design aerodynamic efficiencies. Asshown in FIG. 7 , a plurality of aftercoolers 279 may be employeddownstream of compressors and a small make-up flow 211 may be requiredto compensate for leakage losses in the turbomachinery.

For extensive, efficient turndown it is also desirable that the coldturbine exhaust pressure is maintained well above atmospheric pressure.With such a design, the lowest pressure of the system 200 will remainabove atmospheric at turndown. There is no liquid nitrogen handling forthis system, which adds simplicity. A liquid buffer tank, or multipletanks will probably be needed for control and operation of therefrigerant circuit. The refrigeration output of the present system andmethod is primarily be controlled by modulating the refrigerant flowrate and the nitrogen refrigerator should be modulated to balance therefrigeration load.

Additional Features of the Nitrogen Refrigeration Systems

Additional design features in the present cryogenic refrigerationsystems and methods may prove beneficial to enhance the operability andflexibility of the above-described embodiments. One important feature isthe ability of the cryogenic refrigeration system to handle smallvariations in refrigeration loads, and more particularly variations inthe nitrogen return gas as the refrigeration load changes. For largesupercomputing applications, it is foreseeable if not likely that therefrigeration loads may be in disparate locations within the large datacenters or even at separate facilities. As a result, the flow and othercharacteristics of the nitrogen return gas may also vary due tooperational changes in the refrigeration loads or refrigeration loadcircuits, even if the net total refrigeration load does not change.

A decrease in the nitrogen return gas from the refrigeration loadcircuits may lead to a drop in the pressure of the nitrogen returngas/flash gas line in the nitrogen refrigerator. Depending on thepressure of the low pressure circuit, a relatively minor decrease in thereturn gas flow may cause the pressure of this line to drop belowatmospheric pressure. Any drop in pressure below atmospheric pressureneeds to be avoided so that air incursions into the refrigeration systemand/or refrigeration load circuits does not occur. Even if the lowpressure circuit pressure is higher a decrease in return flow that isextreme enough, or long lasting enough may cause the pressure in the gasreturn circuit to fall too low. Also, if the return gas decreasecontinues for any extended time, the entire refrigeration systempressure will drop, and the refrigeration output will decrease, which inturn may introduce a large instability in the refrigeration system.

To mitigate these problems, variations to the integrated nitrogenrefrigeration systems described with reference to FIGS. 1-5 areillustrated in the integrated nitrogen refrigeration systems of FIGS. 8through 10 . Many of the components and features shown in FIGS. 8-10 arethe same as described above with reference to FIGS. 1-5 and for the sakeof brevity will not be repeated. Rather, the following discussion of theembodiments shown in FIGS. 8-10 focuses on various additional componentsand features.

For example, as seen in FIGS. 8-10 a trim heater 333 is included in therefrigeration load circuits. Specifically, an electric trim heater 333is used to maintain a constant nitrogen return flow of saturated vaporfrom the refrigeration load circuit. The trim heater 333 is showninstalled in the buffer tank 150 but could also be installed in anadditional liquid tank or elsewhere in the nitrogen return gas circuit.To compensate for an unplanned reduction in gas return vapor, the trimheater load would be increased.

Any reduction in gas return vapor may be sensed by a flow meter or apressure transducer in the nitrogen return gas circuit or conduit. Thetrim heater 333 is preferably always on at a low output in order toensure its ability to respond quickly. Another important feature is theability of the cryogenic refrigeration system to handle intentionalchanges in operation of the refrigeration system in an effort to managepower consumption to reduce operating costs of the refrigeration system.An example of such imposing intentional changes in operation isoperation in a modal operating mode that produces and stores excessliquid nitrogen part of the time when power costs are generally lowerand/or consumes the stored liquid nitrogen as required by theapplication and associated refrigeration loads. As discussed above, thismodal operating mode may be advantageous in situations where power costsvary as a function of time and where the refrigerator may be turned downor shut off entirely during high power cost periods. These may beregular “time of day” power cost changes, or more variable time of usepower cost changes driven by electric grid capacity demands.Alternatively, excess liquid nitrogen could be produced by the nitrogenrefrigerator for export as a merchant liquid or for other uses at thecustomer site in addition to meeting the cooling requirements of theintended application.

Variable production of excess liquid nitrogen and/or variableconsumption of liquid nitrogen requires substantial gas storage at thewarm end of the cryogenic refrigeration system. As the refrigerationoutput of the nitrogen refrigerator is modulated, the pressures withinthe refrigeration system will change. The most efficient turndown methodof the nitrogen refrigerator is preferably the same as turndown innitrogen liquefiers. During such turndowns, all the pressure levelswithin the liquefier/refrigerator fall in concert so that the turbinesand the recycle compressor pressure ratios and volumetric flow ratesstay nearly constant. In this way these turbomachines continue tooperate at or near their design point efficiencies. This turndown methodalso enables a very large turndown range. Generally, the turbine inletnozzle positions are fixed in this method. The pressure at the suctionof the low pressure recycle compressor will necessarily decrease whenturndown is affected using this method.

FIG. 9 shows an example refrigeration configuration similar to theembodiment of FIG. 2 but with an optional throttle valve 345 disposed inthe low pressure return circuit 123 near the cold end of the heatexchanger 112 that would be used if the turndown method allowed the lowpressure recycle compressor suction pressure to decrease. Us e of theoptional throttle valve 345 keeps the refrigeration load circuit atconstant pressure and refrigeration temperature, rather than allowing itto fall with the pressure of the low pressure recycle compressor 113.Locating this optional throttle valve 345 at the cold end of the heatexchanger 112 is thermodynamically better than locating the throttlevalve at the warm end of the heat exchanger 112. However, the optionalthrottle valve 345 and installation thereof is generally less costly ifit is configured or located at the warm end. In order to make use ofthis method of turndown, the pressure of the low pressure recyclecompressor 113 must be elevated above atmospheric. The greater degree towhich it is elevated, the greater the extent of efficient turndown thatcan be accomplished. An alternative approach to refrigeration turndownthat will likely be acceptable in most cases is to decrease the pressureratios of the turbines 170, 175 and consequently the recyclecompressor(s) 113, 114 by opening the turbine inlet nozzles. Thisturndown method will be less efficient and less rangeable than theformer method, as all the turbomachinery will move away from theirdesign operating points and design efficiencies. But the low pressurerecycle compressor 113 suction pressure need not decrease.

In either turndown method at least some of the system pressure levelswill decrease, then need to increase again when the refrigeration rateis turned back up. This means that, without any nitrogen gas supply froma nitrogen producing air separation unit, the nitrogen gas must becaptured and then recovered. Very small ranges in capacity can behandled with an externally supplied liquid tank. For larger capacitychanges a receiver, or multiple of receivers 355 (i.e. a receiver bank),as shown in FIG. 9 will capture the nitrogen gas molecules at the warmend. When the refrigeration capacity is being turned down, the systempressures decrease, and nitrogen gas is supplied at pressure from thedischarge of the medium pressure recycle compressor 114 or the dischargeof the warm booster 172 or cold booster 176.

When the capacity is increased, the stored gas in the receiver bank 355is returned via valve 357 to the refrigeration system 100 at the lowestpressure location of the refrigeration system, namely the low pressurerecycle compressor 113 suction end. The high pressure supply of nitrogengas to the receiver bank 355 via valve 358 is optionally from dischargeof the boosters 172, 176, rather than the discharge of the recyclecompressor(s) 114 via valve 359. This is beneficial in that it requiresless receiver bank volume but may require a higher design pressure forthe receiver bank 355.

Another feature of the refrigeration system shown in FIG. 9 is anoptional start-up heater 366. The purpose of the optional start-upheater 366 is to provide nitrogen gas to the refrigeration system uponrestart after a shutdown. During a restart it is conceivable that theintroduction of cold return flow to the nitrogen refrigerator will notbe balanced by a sufficient feed flow from the compressors when therefrigeration system is started up. This would mean that gas exiting thewarm end of the nitrogen refrigerator is unacceptably cold. Thisstart-up heater 366 would provide a relatively low flow of warmednitrogen gas via valve 367 to enable start-up of the nitrogenrefrigerator at low capacity.

For the anticipated normal capacity modulations needed to respond torefrigeration load and typical weather variations, a refrigerationsystem with a receiver bank is likely to provide a satisfactorysolution. If, on the other hand, substantial capacity modulation isexpected or planned, the volume of return gas storage to be provided bythe receiver bank may be excessive. In this case, a nitrogen producingair separation unit 400 is preferably coupled to the nitrogenrefrigeration system as shown in FIG. 10 .

Liquid Air Refrigeration Systems

An alternative to the nitrogen refrigeration system having a largereceiver bank or that requires a nitrogen producing air separation unitmay be to operate the cryogenic refrigeration system with liquid airrather than nitrogen. A liquid air based refrigeration system offers adesign that has many operational advantages and is much moresustainable. Ultimately, the atmosphere provides an infinite gas supplyso that a liquid air based refrigeration system obviates any need for areceiver bank or a gas producing air separation unit. In addition, therangeability of the air-based cryogenic refrigeration system is notlimited by the gas supply and concerns relating to avoiding sub-ambientpressures are no longer relevant.

However, using liquid air as the refrigerant in the integratedrefrigeration system limits the minimum achievable temperature to 82.0 Krather than the of 77.5 K target refrigeration temperature that can beprovided with a liquid nitrogen based refrigeration system. Also, theuse of liquid air could lead to a genuine safety concern due to thepotential for oxygen enrichment that may occur within the system, whichcan generally be avoided with a design that avoids or minimizes ‘dead’legs in the refrigeration circuit, and with operating criteriaspecifying periodic or measurement based liquid drainage from tanks andassociated circuits, including any such ‘dead’ legs.

FIGS. 11-13 show various embodiments of a liquid air based refrigerationsystem 300. Many of the individual components and features shown inFIGS. 11-13 are the same or similar to those described above withreference to FIGS. 8-10 and for the sake of brevity will not berepeated. Rather, the following discussion of the embodiments shown inFIGS. 11-13 focuses on the differences between the liquid air basedintegrated refrigeration system and the nitrogen based integratedrefrigeration systems 100 described with reference to FIGS. 8-10 .

FIG. 11 shows a liquid air based refrigeration system based similar tothat shown in FIG. 2 and FIG. 9 , although any of the embodiments shownin FIGS. 2-5 and 8-10 could also be modified to use liquid air.Comparing the embodiment of FIG. 11 to that of FIGS. 2 and 9 , theoptional cold compressor 119 of the nitrogen based system is removed asit is not needed even when the design calls for the coldestrefrigeration temperatures possible. Also, there will be no liquidwithdrawal from the optional storage tank 160.

The key additions to the embodiment of the liquid air based system shownin FIG. 11 compared to that of the liquid nitrogen based system of FIGS.2 and is the addition of an air pre-purifier 500 after the low pressurerecycle compressor 113 to remove contaminants that would freeze outcontaminants such as carbon dioxide, moisture, oxides of nitrogen, andsome heavy hydrocarbons that are undesirable for process safety. Thepre-purifier 500 is preferably a temperature swing adsorption (TSA)based unit that minimizes the required regeneration flow. Theregeneration heater 502 is most likely electrically heated, although itcould as well be natural gas fired or steam heated. The bypass circuit504 around the heater 502 is for the cooling step that is a normal partof the TSA pre-purifier operating cycle. A portion of the cleanedpre-purifier outlet stream 506 is returned for regeneration. Lowpressure return gas from the refrigerator warm end could be consideredas a gas source for the regeneration flow. However, the pre-purifieroutlet gas is a better choice.

FIG. 12 shows yet another alternative embodiment of the liquid air basedrefrigeration system 300 configured for applications that has a somewhathigher target refrigeration temperature and where the low pressurereturn stream is elevated in pressure. In this case air compressionstages are required to feed fresh air into the system, when it isneeded. The location of the pre-purifier 500A is shown to be after themain air compressor 513 and upstream of the return of the low pressurestream. This is based on a design scenario where the maximum feed airflow is relatively low compared to the low pressure return flow.Alternatively, the pre-purifier 500B can be located after the lowpressure recycle compressor 113. The choice of location depends on themaximum feed air flow, the air compressor discharge pressure, andwhether the liquefier turndown method allows the low pressure circuit todecrease in pressure. These factors affect the sizing of thepre-purifier 500A, 500B. For example, if the air compressor dischargepressure is relatively low and the feed air flow is high enough thatadditional compression in the low pressure recycle compressor wouldresult in more moisture condensation and a lower volumetric flow, thenthe alternative pre-purifier 500B location would be preferred. Also, ifthe turndown method entails reducing the pressure of the low pressurereturn gas circuit, that would mean the pre-purifier would operate atlower pressure upon recharging the system with air and increasing therefrigeration capacity. This would require larger beds or throttling ofthe air after the pre-purifier. So, this would also tend to make thealternative pre-purifier 500B location preferable. It is not shown inthe drawing, but there also will need to be a vent of air to atmospherein the system so that the liquid air based refrigeration system 300pressures can be reduced when the refrigerator capacity is beingunloaded. To minimize the wasted power during this operation the ventingstream should be at the lowest pressure possible. It can be locatedafter the low pressure recycle compressor, or better, after the firststage of the low pressure recycle compressor.

When the refrigeration temperature is high enough, the return gas fromthe refrigeration load will be similar in pressure to the design exhaustpressure of the cold turbine. In this case a separate low pressurereturn stream is not needed. For a nitrogen based refrigerator thisoccurs when the refrigeration temperature is about 95-97 K, and higher.For an air based refrigerator this will occur when the refrigerationtemperature is at least about 99.5 K-101.5 K. FIG. 13 shows a liquid airbased refrigeration system 300 with a warm enough refrigerationtemperature so that there is no low pressure return stream (See 123, 345in FIGS. 2, 4, 5, 9 and 10 ). In this case, the air compression and airpre-purification are always entirely separate from the gas recyclecircuit.

While the present systems and methods for cryogenic refrigeration havebeen described with reference to several preferred embodiments, it isunderstood that numerous additions, changes and omissions can be madewithout departing from the spirit and scope of the present system andmethod as set forth in the appended claims. Specifically, alternativecold end configurations of the integrated nitrogen refrigeration systemare contemplated. For example, the buffer tank may be used for liquidnitrogen addition or liquid nitrogen manufacture in lieu of a separatestorage tank. In such arrangement, the buffer tank would probably besized larger to satisfy the dual functions. Another contemplated variantwould be to combine the separator and buffer tank function in a singleliquid vessel. Some further contemplated alternatives includearrangements where the optional liquid turbine is loaded by a compressorthat raises the pressure of another stream or it could be loaded by anenergy dissipating brake instead of loading by a generator. Of course,the optional liquid turbine is a power saving feature that may be usedin applications where the additional capital costs are justified.

The contemplated designs of the presently disclosed refrigerationsystems and methods are readily scaleable in size by increasing ordecreasing the size of the various compressor(s), turbine(s), heatexchanger(s) and associate equipment and piping circuits. It is expectedthat the disclosed cryogenic refrigeration systems would be suitable foruse in applications providing between about 20 kW of refrigeration toabout 2000 kW of refrigeration or more to the refrigeration loadcircuits.

What is claimed is:
 1. A liquid nitrogen based integrated refrigerationsystem comprising: a nitrogen refrigerator having one or more recyclecompressors, a warm booster compressor, a cold booster compressor, awarm turbine, a cold turbine, and a heat exchanger with at least onecooling passage and at least one recycle passage; wherein the nitrogenrefrigerator is configured to receive a source of nitrogen gas and anitrogen return stream and produce a nitrogen refrigerant stream; arefrigeration load circuit having an expansion valve or a liquidturbine, a separator, a buffer tank, and a refrigeration load; whereinthe refrigeration load circuit is configured to: (a) receive thenitrogen refrigerant stream; (b) expand the nitrogen refrigerant streamin the expansion valve or the liquid turbine; (c) separate the expandednitrogen refrigerant stream in the separator into a liquid portion and avapor portion; (d) cool a refrigeration load with the liquid portion ofthe expanded nitrogen refrigerant stream while vaporizing the liquidportion of the expanded nitrogen refrigerant stream; and (e) return thevaporized stream and the vapor portion of the nitrogen refrigerantstream as the nitrogen return stream to the nitrogen refrigerator. 2.The liquid nitrogen based integrated refrigeration system of claim 1,further comprising an optional cold compressor disposed downstream ofthe refrigeration load and configured to boost the pressure of thenitrogen return stream.
 3. The liquid nitrogen based integratedrefrigeration system of claim 1, wherein the heat exchanger isconfigured with: (i) one cooling passage and two recycle passages; (ii)one cooling passage and one recycle passage; (iii) two cooling passagesand two recycle passages; or (iv) two cooling passages and three recyclepassages.
 4. The liquid nitrogen based integrated refrigeration systemof claim 1, wherein the one or more recycle compressors furthercomprises: (i) a low pressure recycle compressor and a medium pressurerecycle compressor; (ii) a low pressure recycle compressor, a mediumpressure recycle compressor, and a high pressure recycle compressor. 5.The liquid nitrogen based integrated refrigeration system of claim 1,wherein the warm booster and the cold booster are arranged is series. 6.The liquid nitrogen based integrated refrigeration system of claim 1,wherein the warm booster and the cold booster are arranged is parallel.7. The liquid nitrogen based integrated refrigeration system of claim 1,wherein the warm turbine is a warm boosted loaded turbine operativelycoupled to the warm booster and the cold turbine is a cold boostedloaded turbine operatively coupled to the cold booster.
 8. The liquidnitrogen based integrated refrigeration system of claim 1, wherein thewarm turbine and the cold turbine are configured as radial inflowturbines and have a pressure ratio of between about 8.5 to 10.0.
 9. Theliquid nitrogen based integrated refrigeration system of claim 1,further comprising an optional liquid storage tank disposed in operativeassociation with the refrigeration load circuit.
 10. The liquid nitrogenbased integrated refrigeration system of claim 1, further comprising atrim heater disposed within the buffer tank and configured to modulatethe nitrogen return stream recycled to the heat exchanger.
 11. Theliquid nitrogen based integrated refrigeration system of claim 1 furthercomprising an optional throttle valve configured to throttle thenitrogen return stream.
 12. The liquid nitrogen based integratedrefrigeration system of claim 1 further comprising an optional start-upheater.
 13. The liquid nitrogen based integrated refrigeration system ofclaim 1 further comprising one or more gas receivers configured tomodulate the nitrogen gas fed to the nitrogen refrigerator.
 14. Theliquid nitrogen based integrated refrigeration system of claim 1 whereinthe refrigeration system is operatively coupled to a nitrogen producingair separation unit to control the nitrogen gas fed to the nitrogenrefrigerator.
 15. A liquid air based integrated refrigeration systemcomprising: an air intake system having a compressor and a pre-purifier;a refrigerator having one or more recycle compressors, a warm boostercompressor, a cold booster compressor, a warm turbine, a cold turbine,and a heat exchanger with at least one cooling passage and at least onerecycle passage; wherein the refrigerator is configured to receive asource of compressed, pre-purified air and a cold return stream andproduce a liquid air refrigerant stream; a refrigeration load circuithaving an expansion valve or a liquid turbine, a separator, a buffertank, and a refrigeration load; wherein the refrigeration load circuitis configured to: (a) receive the liquid air refrigerant stream; (b)expand the liquid air refrigerant stream in the expansion valve or theliquid turbine; (c) separate the expanded refrigerant stream in theseparator into a liquid portion and a vapor portion; (d) cool arefrigeration load with the liquid portion of the expanded refrigerantstream while vaporizing the liquid portion of the expanded refrigerantstream; and (e) return the vaporized stream and the vapor portion of theexpanded refrigerant stream as the cold return stream to therefrigerator.